Method for isolating amines from aqueous solutions

ABSTRACT

A process for the isolation of amines from aqueous solutions by continuous distillation of aqueous solutions of the amines by a two-pressure method of rectifying aqueous solutions of amines which form with water an azeotrop, the composition of which is pressure-dependent, in two columns at different pressures provided that: a) the pressure in the second column is set to a value at least 0.5 bar higher than in the first column, b) the aqueous amine solution is fed, as a side stream, to the first column, and water and portions which boil at a higher temperature than the amine or which are non-volatile are removed at the base of the column, c) a mixture of amine and water is withdrawn at the head of the first column and fed, as a side stream, to the second column, d) an azeotrope of water and amine is withdrawn at the head of the second column and recycled to the first column, and (e) the purified amine is discharged at the base of the second column.

The invention relates to a process for the isolation of amines fromaqueous solutions by continuous distillation of aqueous solutions of theamines by a two-pressure method of rectifying homogeneous azeotropicbinary mixtures.

Two liquids which are completely soluble in each other and formazeotropes whose composition is pressure-dependent, can be separated bydistillation without the assistance of an additive. Such a separationprocess involving rectification of liquid blends in two columns operatedat different pressures are referred to as two-pressure processes, cfKlaus Sattler, “Thermische Trennverfahren, Grundlagen, Auslegung,Apparate”, 2nd Edition, Verlag Chemie, Weinheim, New York, Basel,Cambridge, Tokyo, 1995, pp. 147 et seq.

In practice there are obtained aqueous solutions of amines from whichthe amines have to be isolated in anhydrous form. Since the amines forman azeotrope with water, they cannot be readily isolated from aqueoussolutions in anhydrous form by distillation. The recovery of pure aminesfrom aqueous, salty solutions is an important factor when measuring theeconomical value of processes in which the amines are used as bases. Inthis process the amines must be recovered from the aqueous solutions oftheir ammonium salts formed during the reaction.

It is an object of the present invention to provide a process for theisolation of amines in substantially anhydrous form from aqueoussolutions of said amines.

This object is achieved in a process for the isolation of amines fromaqueous solutions by continuous distillation of aqueous solutions of theamines by a two-pressure method of rectifying homogeneous azeotropicbinary mixtures, when aqueous solutions of amines forming an azeotropwith water, the composition of which is pressure-dependent, aredistilled in two columns at different pressures, wherein

-   -   a) the pressure in the second column is set to a value at least        0.5 bar higher than in the first column,    -   b) the aqueous amine solution is fed as a side stream to the        first column, and water and portions which boil at a temperature        higher than the boiling point of the amines or which are        non-volatile are removed at the base of the column,    -   c) a mixture of amine and water is withdrawn at the head of the        first column and is passed as a side stream to the second        column,    -   d) an azeotrope of water and amine is withdrawn at the head of        the second column and is recycled to the first column, and    -   e) the purified amines are discharged at the base of the second        column.

In a preferred embodiment of the process of the invention, the energyrequired to vaporize the azeotrope in the first column is introduced bypassing steam into the lower part of this column and removing water andportions which boil at a higher temperature than the amines or which arenon-volatile from the base of the column and removing highly volatileportions boiling at a lower temperature than the amines from the upperpart of the first column. The pressure in the first column is set, forexample, to values of 10 mbar to 10 bar and is usually from 0.1 to 3bar. Particular preference is given to a mode of operation involving theuse of atmospheric pressure in the first column.

In the preferred embodiment of the invention, the pressure in the secondcolumn is set to a value at least 1 bar higher than in the first column.In the particularly preferred embodiment of the invention the pressurein the second column is at least 6 bar higher than in the first column.The pressures and other conditions used for distillation (eg,temperature of the heat transfer medium such as cooling water, brine, orsteam, the diameter of the columns and the size of the return streambetween the columns) are preferably such that with regard to processengineering and economical aspects an optimal solution for the isolationof the amines is achieved.

According to the present invention, all amines which form with water anazeotrop whose composition is pressure-dependent, can be isolated in theprocess of the invention in virtually anhydrous form. The water contentof the amines isolated from the aqueous solutions is, for example, from10 to 200 ppm, and preferably from 20 to 60 ppm. In the process of theinvention, the amines used are preferably N,N-dimethylisopropylamine,methyl-n-butylamine, or n-butylamine. Particular preference is given tothe use of N,N-dimethylisopropylamine.

Such amines are used, for example, in substantially anhydrous form inthe aforementioned process for the production of, say, alkyl ketenedimers. The resultant ammonium salts are separated and treated withaqueous solutions of bases in order to liberate the amines from thecorresponding ammonium salts. Suitable bases are, for example, sodiumhydroxide solution, potash lye, calcium hydroxide, and barium hydroxide.

Thus, for example, aqueous sodium hydroxide solution is added to anaqueous solution of N,N-dimethylisopropylammonium hydrochloride in aconcentration such as to give a pH of the aqueous ammonium salt solutionof at least 11. By this means N,N-dimethylisopropylamine is liberatedfrom the ammonium salt solution and sodium chloride formed. This mixturecan then be continuously distilled by the method of the invention toisolate N,N-dimethylisopropylamine in virtually anhydrous form.

Aqueous solutions of the amines, which may optionally still containimpurities, such as low-boiling components or neutral salts such assodium chloride, can be distilled, for example, by the two-pressuremethod as follows: an aqueous, neutral salt-containing solution ofamines which form with water an azeotrope whose composition ispressure-dependent is continuously fed as a side stream to a firstcolumn operated under atmospheric pressure. The side stream can, forexample, be fed to the center of the column. The column has separatingdevices comprising plates, preferably sieve trays. The number oftheoretical plates of the column is, eg, from 5 to 15, preferably from 8to 12. The aqueous amine solution to be distilled and fed to the firstcolumn, can have a temperature ranging from 20° to 80°, preferably from45° to 60° C. have. The energy necessary for distillation can be appliedto the system with the aid of an evaporator at the base of the column oralternatively by the introduction of steam. Direct introduction of steaminto the column, eg, at the lower end the column, is preferred. Thisavoids the formation of caked material in the column. The bottoms of thefirst column contain water and portions which boil at a highertemperature than the amines or are non-volatile, such as neutral salts.The overheads from the first column, comprising a mixture of amine andwater (eg, 95 wt % of amine and 5 wt % of water), are continuouslywithdrawn and passed as a side stream to, eg, the upper half of thesecond column, in which the pressure is 7 bar, for example. An almostanhydrous amine is isolated at the bottom of the second column, likewisecontinuously. At the head of the second column there is continuouslywithdrawn an azeotrope of water and amine (eg, 89 wt % of amine and 11wt % of water) which is recycled, for example, to the upper half of thefirst column. Highly volatile portions may optionally be continuouslyremoved from the upper part of the first column. The virtually anhydrousamine can be reused, for example, in the synthesis of alkyl ketenedimers from carboxylic chlorides.

The reflux ratio is, for example, in the first column from 0.5 to 3,preferably from 0.5 to 2 and in the second column from 1 to 4,preferably from 1 to 2.

EXAMPLE

The dimethylisopropylammmonium chloride formed during the production ofalkyl ketene dimer from a long-chain carboxylic chloride (eg, stearoylchloride) and N,N-dimethylisopropylamine in the form of a ca 50 wt %strength aqueous solution is set to pH 11 by the addition of 25 wt %strength aqueous sodium hydroxide solution and is heated to atemperature of 50° C. and continuously fed, as a side stream, to acolumn having 25 sieve trays and operated under atmospheric pressure.Heating is effected by continuous introduction of live steam having atemperature of ca 150° C. into the lower region of the column. The steamrate is ca 20% of the rate at which the amine solution is fed to thefirst column. From the base of the first column there are removed water,neutral salts such as sodium chloride, excess sodium hydroxide solution,and ca 10 ppm of N,N-dimethylisopropylamine. At the head the columnthere is formed a mixture of almost azeotropic composition (95 wt % ofN,N-dimethylisopropylamine and 5 wt % of water). The condensed vaporsare passed, as approximately equal partial streams, back to the firstcolumn and continuously, as a feed side stream, to the upper half of thesecond column, which is operated under a pressure of 7 bar absolute andhas 30 sieve trays.

In the enriching zone of the first column there is a side flue, throughwhich impurities are continuously withdrawn which are more volatile thanwater.

The bottom of the second column is heated by means of an evaporator. Atthe bottom of the second column, pure N,N-dimethylisopropylaminecontaining ca 40 ppm of water is continuously withdrawn. It can bedirectly reused, eg, for the synthesis of alkyl ketene dimers ofcarboxylic chlorides.

At the head of the second column there is obtained an azeotropecontaining 89 wt % of N,N-dimethylisopropylamine and 11 wt % of water.The reflux ratio in the second column is 1.5. The azeotrope formed atthe head of the second column is fed to the first column continuously asa side stream above the point at which the aqueousN,N-dimethylisopropylamine solution is metered. The condenser of thesecond column is degassed into the enriching section of the firstcolumn, which is operated under standard pressure.

1. A process for the isolation of an amine from an aqueous solution by continuous distillation of an aqueous solution of the amine by a two-pressure method of rectifying homogeneous azeotropic binary mixtures, wherein an aqueous solution of an amine which forms with water an azeotrop, the composition of which is pressure-dependent, is distilled in two columns at different pressures provided that: a) the pressure in the second column is set to a value at least 0.5 bar higher than in the first column, b) the aqueous amine solution is fed, as a side stream, to the first column, and water and portions which boil at a higher temperature than the amine or which are non-volatile are removed at the base of the column, c) a mixture of amine and water is withdrawn at the head of the first column and fed, as a side stream, to the second column, d) an azeotrope of water and amine is withdrawn at the head of the second column and recycled to the first column, and (e) the purified amine is discharged at the base of the second column.
 2. A process as defined in claim 1, wherein, in the first column, the energy necessary for vaporization of the azeotrope is introduced by feeding in steam to the lower part of this column, and water and portions which boil at a higher temperature than the amine or which are non-volatile are removed at the bottom of the column, and highly volatile portions which boil at a lower temperature than the amine are removed from the upper part of the first column.
 3. A process as defined in claim 1, wherein the pressure in the first column is set to a value of from 10 mbar to 10 bar.
 4. A process as defined in claim 1, wherein the pressure in the first column is set to atmospheric pressure.
 5. A process as defined in claim 1, wherein the pressure in the second column is set to a value at least 6 bar higher than in the first column.
 6. A process as defined in claim 1, wherein the pressure in the second column is set to a value at least 1 bar higher than in the first column.
 7. A process as defined in claim 1, wherein the amine used is N,N-dimethylisopropylamine, methyl-n-butylamine or n-butylamine.
 8. A process as defined in claim 1, wherein, for said continuous distillation, an aqueous solution of an amine is used which is obtained by liberating the amine from an aqueous amine salt solution with a base.
 9. A process as defined in claim 1, wherein the amine used is N,N-dimethylisopropylamine.
 10. A process as defined in claim 1, wherein the separating devices used are plates.
 11. A process as defined in claim 1, wherein the separating devices used are sieve trays. 